The present invention relates to workpiece hold-down devices for machines in which the machining tool is movable in a plane parallel to a workpiece consisting of a sheet or sheets of stock material, for machining the workpiece in a desired pattern by means of successive concurrent orthogonal tool movements.
While the principles of the invention can be used in a wide variety of applications in which there is a need to hold a workpiece in place while being machined, the embodiment of the invention that is disclosed herein is suited for retaining a stack of relatively thin workpieces, such as a stack of sheet metal, during simultaneous routing of the stack on a numerically controlled (N/C), gantry-type of router. For example, in the mass production of metal parts, it is often desirable to simultaneously cut a large number of relatively thin sheets into a desired two-dimensional pattern. For this purpose, a stack of ten or twenty relatively thin metal sheets may be laid on a router bed, and secured to the bed by such means as clamps, temporary hold-down bolts or screws. The sheet metal stack, once secured in this manner, is then machined by a router in which the routing tool penetrates the entire thickness of the stack, and is moved parallel to the upper surface of the stack, in a controlled fashion to simultaneously rout all of the sheets of the stack to the desired pattern.
The equipment usually employed for this kind of routing includes a broken-arm router and a preshaped template which is disposed as the top layer of the stack, and which serves to guide the spindle assembly and the associated routing tool to form the desired pattern. The template, in addition to controlling the outline of the part, also provides an effective means for retaining the stack of metal sheets in a clamped-up state. The template is laid on top of the stack of unformed metal sheets, and holes are drilled through the sheets at guide openings provided on the template. Then lag screws are mounted in these openings and tightened down into an underlying sheet of plywood that is affixed to the bed of the machine. The number and location of the lag screws are selected so that the layers of sheet metal are held together with such integrity that there is no opportunity for cutting chips, generated during the routing operation, to work their way in between the sheets adjacent the path of the cutting tool. The template, which is usually made of a thicker material than the individual sheets, and is thus more rigid, distributes the force exerted by the individual, hold-down lag screws, uniformly about the surface area of the stack so that adequate stack clamp-up is provided not only immediately adjacent the lag screws, but also at all locations between such screws. With such a clamp-up, any residual chips left on the stack cannot be permitted to get in between the sheets and damage the surface quality of the part.
The use of broken-arm routers and templates, has heretofore provided an effective technique for maintaining the surface quality of stack routed parts. Now, however, in an effort to reduce the overall cost per unit of stack-routed parts, operations previously performed on a broken-arm router are now being carried out on gantry-type, N/C routers. Such N/C routers eliminate the labor intensive template making and clamp-up procedure required with the former machines and offer the flexibility of being able to rout a variety of different parts from a stack of large sheet metal blanks, and to automatically determine the proper location and orientation of the various parts for maximum utilization of the available surface area of the blank stack. Once the particular part patterns are entered into the data processor of the numerical controlled machine, and after the processor has determined the most efficient location and orientation for the selected patterns, the router spindle assembly is driven, successively or concurrently, in orthogonally related directions over the upper surface of the sheet metal stack, causing the router tool to cut out the desired patterns.
While it will be appreciated that the N/C router offers significant advantages in terms of flexibility and efficient utilization of material, the elimination of the part control templates that are used on the broken-arm routers, results in the loss of a convenient and effective means of stack clamp-up needed to prevent surface damage to the parts caused by cut chips entering gaps between the sheets. A solution to this problem has not been readily found, and a number of unsuccessful efforts to overcome this difficulty were proposed and tried before realizing the principles of the present invention. One such prior, unsuccessful attempt was to provide a sliding pressure-foot that slid over the upper surface of the stack to accommodate the relative movement between the router spindle and the stack. However, for effective stack retention, so much downward pressure had to be exerted on the sliding foot that the upper surface of the top sheet was intolerably scratched. Additionally, the sliding pressure-foot, when subjected to the necessary hold-down pressure, was unable to ride up and over the heads of lag screws, a few of which are still needed on the N/C stack routing machine for holding the stack against shear, and at a fixed reference with respect to the machine bed. Similarly, an attempt to use air pressure bearings to exert the necessary downward force on the stack, while minimizing friction and associated surface abrasion, was unsuccessful because of the inability of the air bearings to negotiate the heads of the clamp-up lag screws. Also, air pressure loss was excessive because of escapement of air through the slots necessarily cut in the stack by the router tool.
Other proposed solutions to the stack clamp-up problem have included a process of submerging the metal sheets in water and freezing the submerged stack prior to machining; and a process of applying adhesive between the contacting surfaces of the sheets and letting the adhesive set up prior to routing. Both these processes proved too costly and significantly detracted from the other cost-effective benefits of numerically controlled routing.
Accordingly, one object of the invention to provide a workpiece hold-down apparatus for machines of the type characterized above, wherein the hold-down apparatus is effective in applying a compressive hold-down force on a stack of relatively thin workpiece sheets and for concentrating such hold-down force in a localized region immediately surrounding the machining tool to prevent chips generated by the machining operation from entering gaps between the individual sheets.
Another object of the invention is to provide a hold-down apparatus for a workpiece consisting of a stack of metal sheets that are to be simultaneously routed to a predetermined two-dimensional pattern by a gantry-type router, in which a router spindle assembly is movable on and by a gantry in successive and/or concurrent orthogonal directions in a plane parallel to the stack of sheet metal.
Still another object of the invention is to provide a hold-down apparatus for machines in which the machining tool is moved in controlled patterns relative to the surface of sheet material by a numerically controlled processor so as to machine the material in a preselected pattern.